1. Field of the Invention
The present invention relates to an image sensor, and particularly to an image sensor using an avalanche photodiode, a single-plate color image sensor and an electronic device.
2. Description of the Related Art
With an increase in the market of electronic devices, such as digital cameras, portable telephones with a built-in camera, and the like, the development of image sensors is being actively pursued. In particular, there is intensified competition in the development of a metal-oxide semiconductor (MOS) image sensor, which can be manufactured by typical semiconductor manufacturing processes, such as complementary MOS (CMOS) processes.
In the development trend to increase the number of pixels and decrease optical size of the MOS image sensors, miniaturization of a pixel size of the MOS image sensor is an important subject. A pixel size of about 2.2 μm has been achieved for a commercial product. A pixel size of about 1.7 μm has been reported at an academic conference. Moreover, many manufacturers have announced schedules for the mass production of MOS image sensors having a 1.7 μm pixel. Similarly, conventional mainstream charge-coupled device (CCD) image sensors are also advancing the development of miniaturized pixels.
In association with the miniaturization of the pixel, it is impossible to avoid a tendency of decreased sensitivity of the image sensors. That is, the area of a light sensitive element, such as a photodiode and the like, may be decreased along with the miniaturization of the pixel. Since the intensity of an incident light for each pixel is decreased, the amount of a signal electric charge, which is generated by photoelectric conversion and accumulated in a pixel, may be decreased.
As one method for obtaining a large output signal from few signal charges, a technique for multiplying the signal charges by avalanche multiplication is known. For example, an image pickup tube using an amorphous selenium (a-Se) photoconductive film for an avalanche multiplication layer has been reported (refer to Tanioka, et al., IEEE Electron Device Letters, September 1984, Vol. EDL-8, p. 392-394). Also, an image sensor stacking a-Se avalanche multiplication layers has been reported (refer to Takiguchi, et al., IEEE Transactions on Electron Device, October 1997, Vol. 44, No. 10, p. 1783).
In the document of Takiguchi, et al., a high-gain avalanche rushing amorphous photoconductor (HARP) film having a thickness of about 0.5 μm is connected to a CMOS sensor through a bump having a diameter of about 5 μm to form a hybrid image sensor. It has been reported that, by applying a voltage of about 75 V to the HARP film, a gain of about 10 times can be achieved.
However, when using the multiplication film made of the material without silicon (Si), the hybrid structure and the bump are required, as mentioned above. Thus, the use of the avalanche multiplication layer is not necessarily suitable for miniaturization of the pixel.
Furthermore, a normal avalanche multiplication layer has a longitudinal structure vertical to a semiconductor substrate surface (refer to JP-A 2000-323747 (KOKAI)). In the avalanche multiplication layer, an electrical field for generating avalanche multiplication is parallel to the incident light. A short wavelength light is absorbed in the vicinity of the semiconductor substrate surface. On the other hand, a long wavelength light is transmitted to a deeper position from the semiconductor substrate surface. As a result, the avalanche multiplication factor is different depending on the wavelength of a light absorbed in the avalanche multiplication layer. Thus, there is a problem in that a spectral sensitivity of the avalanche multiplication layer is changed.
Also, a lateral trench optical detector that can carry out a high speed response has been proposed (refer to U.S. Pat. No. 6,177,289). In U.S. Pat. No. 6,177,289, an n-type trench region and a p-type trench region are alternately formed in each of a plurality of trenches formed in a semiconductor substrate, so as to form a lateral PIN photodiode.
Furthermore, in a single-plate color image sensor, in which the three primary colors of red, green and blue (RGB) in a light are received by a single sensor, it is necessary to form a color filter above a photodiode, in order to obtain pixels having different spectral sensitivities. Because of the existence of the color filter, the distance between a photodiode surface and a micro lens may be increased. Therefore, the light collection efficiency of the micro lens may be decreased, to decrease the sensitivity.